WO2002096654A1

WO2002096654A1 - Inkjet printer with pigmented ink

Info

Publication number: WO2002096654A1
Application number: PCT/US2002/015529
Authority: WO
Inventors: Alan L. Hudd; Philip G. Bentley
Original assignee: 3M Innovative Properties Company
Priority date: 2001-05-30
Filing date: 2002-05-13
Publication date: 2002-12-05
Also published as: TWI248878B; GB0113093D0

Abstract

A drop on demand inkjet printer for use with heavily pigmented inks, and a method for its use with such inks, comprises an ink reservoir (14, 32), a print head (10) having an array of capillary channels each with an ejection nozzle (11), and controlled pulsing means associated with each of the capillary channels. The printer is characterized by means for maintaining a continuous flow of ink from the reservoir (14, 32) through the print head (10) to inhibit settling of solids out of suspension. The printer preferably includes two ink reservoirs, one located at a higher level than the other and both located at a lower level than the print head such that the height difference (AH) between their ink levels provides a siphon to draw ink continuously to and through the print head.

Description

INKJET PRINTER WITH PIGMENTED INK

Field of the Invention

This invention relates to inkjet printing, in particular to a print head unit for use with heavily pigmented inks. The invention also relates to a process for the supply of such inks to an inkjet print head unit.

Background of the Invention

A wide range of inkjet printers is available commercially, from single-user desktop units through more rugged multiple-user office units to industrial systems. Operationally they are of two different types: continuous inkjet (CU) and drop on demand (DOD). The present invention is particularly concerned with industrial units of the latter type, in which ink from a reservoir passes through a feed channel to a static print head having a face plate with an array of ejection nozzles each fed though fine capillary channels, typically of 30- 40 [tm internal diameter. Ink droplets to form a desired image on a substrate are ejected from the nozzles by controlled energy pulses induced by piezoelectric or thermal elements associated with the respective capillaries.

At the start of a typical printing operation ink is fed from the reservoir to the print head both to prime the capillaries and to purge from the nozzles any air bubbles or solid or viscous residues of previous operations. When printing commences a flow of ink is maintained to the capillaries to replace the ink ejected as droplets. The ink is, delivered to the capillaries at a relatively low pressure so as to maintain a slight negative pressure at the nozzles, typically 1 to 2 in. water gauge (0.25 to 0.5 kPa), and thereby avoid emission of ink from the nozzles other than under the action of the controlled energy pulses. Substrate materials to which the ink droplets are applied include ceramics or glass; metals such as aluminium, stainless steel and tin plate; paper, from cardboard and plain copier grades to high quality coated grades; and plastics such as polyacetals, polyesters, polyethylenes and polystyrenes. The substrate may be simply in sheet form, or be shaped as in bottles, cans or other containers. A similarly wide range of images can be applied to the substrate according to the required duties and the complexity of the print head. Possible images include lettering, numerals, figures, photographs, pictures, logos, identifying marks, "sell-by" and "use by" dates, batch numbers, address details and general text. The required degree of precision of the image depends on the duty but the demand in almost every case is for a high speed of application and thus a high frequency of droplet ejection. The large number of possible combinations of printer type, substrate and required image has led to a wide variety of ink types being developed for specific duties. The ink is basically a marker dispersed in a solvent medium but various additives may also be needed, for example dispersants, conductivity controllers or adhesion promoters. In general the marker may be selected from one or more dyes or solid particulate materials, in solution or suspension in the solvent. The markers may be of a type that imparts to the deposited image a coloration which is visible in daylight or conventional artificial lighting, or may have no marked colour in daylight or conventional artificial lighting but reveal their colour under special lighting, for example from an ultra-violet or LED source. Alternatively the markers may be pigments or additives which reveal their presence under the application of magnetic, electronic or spectroscopic means. For example a pigment or an additive with magnetic properties provides a magnetically detectable image.

The combination in the ink of its viscosity, solids content and size and weight of individual particles determines its flow properties and hence the reliability of the printing operation. A particular problem arises if the ink is heavily loaded, either in terms of solids content or in containing dense or heavy particles, in that the viscosity of the ink reaches levels which interfere with the free flow of ink. An additional problem arises with dense or heavy particles, for example heavy magnetic particles, as these tend to settle out of suspension before reaching the ejection nozzle, leading to unsatisfactory image quality or blockage of the feed capillaries. A related problem with such particles is that the ejected droplets may not be uniform, for example in terms of size, shape or velocity, which can lead to the formation of images that are sub-optimal.

The present invention has the objective of providing inkjet printing that avoids these flow, quality and reliability problems. Summary of the Invention

The present invention is specifically concerned with inks containing suspended solid markers and any solid additives. It is most particularly concerned with solids that tend to settle out of suspension, either because of a high proportion of solids or because they contain individual particles of high density.

According to the present invention there is provided a drop on demand inkjet printer comprising a reservoir for ink containing suspended solids, a print head having an array of capillary channels each with an ejection nozzle, and controlled pulsing means associated with each of the capillary channels, in which the printer further comprises a continuous ink-flow subsystem for maintaining a continuous flow of ink from the reservoir through the print head to inhibit settling of solids out of suspension.

The invention further provides a method of drop on demand inkjet printing employing an ink containing suspended solids, in which the ink is fed from a reservoir to a print head having an array of capillary channels each with an ejection nozzle and associated controlled pulsing means, in which a continuous flow of ink from the reservoir is maintained through the print head to inhibit settling of solids out of suspension.

The continuous flow of the ink has been found to have the intended effect of keeping the ink solids in suspension. This greatly assists the free flow of ink, facilitating the continuous smooth delivery to the nozzles of ink of homogeneous quality which in tarn ensures a consistent quality for the ejected droplets and thus the image to be formed. It also makes possible in DOD printing the use of otherwise "unstable" inks, for example inks with an unusually high solids content or inks containing large or high density solids with a tendency to settle out. Use of the invention thereby considerably extends the types of ink that can successfully be used in DOD printing. A further advantage of the continuous flow through the print head is that ink can be constantly recycled to the reservoir, thereby providing with the print head and associated piping a continuous ink flow path. An agitator is preferably included in this flow path to assist in maintaining the solids in suspension or in re-dispersing any solids which settle between printing operations. The agitator may include one or more of static mixers, direct drive impellers, indirect-drive impellers, .and vibration inducers. In one simple form the static mixer(s) may be a plurality of vanes inside a fluid chamber in the flow path. Direct drive impellers are effective but have the disadvantage of requiring a drive shaft through a component of the flow path and thus creating the possibility of ink leakage. An indirect drive impeller, for example employing a free magnet inside a reservoir and a driven magnet outside the reservoir, avoid this disadvantage. Vibration inducers, for example applied to a reservoir, also avoid this disadvantage but care must be taken in their location so that they do not cause vibration and consequent de-priming of the print head.

Filters are preferably disposed in the system to extract oversize or foreign particles therefrom or to prevent oversize or foreign particles from getting into the ink from the atmosphere. The presence of such particles may otherwise interfere with the free flow of ink to and through the ejection nozzles. Suitable locations for filters in direct contact with the ink to extract particles therefrom are, for example, at the base of the reservoirs, or in lines leading to the print head or to the reservoirs. Suitable locations for filters to prevent particles from getting into the ink are in vent lines on the reservoirs. Filters in contact with the ink may for example be mesh filters, with a mesh size corresponding to the size of particles to be removed. Filters in a vent line may for example be mesh filters, porous paper filters or glass fibre filters, the latter being especially suitable for dust removal.

It may be advantageous to include flow meters in the ink flow path. Monitoring the flow at the respective points permits the flow pattern around the system to be adjusted to optimum levels.

Brief Description of the Drawings

Fig. 1 is a schematic view of a printer using a continuous flow system.

Description of the Preferred Embodiments) Examples of types of pigments used in inkjet printer inks are metallic flakes, inorganic materials comprising ferrites and other metal oxides, including oxides of transition and rare earth metals; organo-metallic complexes; and organic materials, including high molecular weight aromatic compounds such as anthraquinones, aryl amides and quinacridones. Specific examples of commonly used pigments include magnetite, barium ferrite, strontium ferrite, iron oxide, titanium dioxide, copper phthalocyanine and carbon black. Two crucial properties of the ink are its viscosity and surface tension. The viscosity should be relatively low, typically in the range 2 to 20 cP (0.002 to 0.02 Pas) at the operating temperature of the print head, such that even when loaded with pigment and additives the ink can pass freely through the flow channels in the print head. In some instances it may be appropriate to reduce the viscosity by increasing the operating temperature of the print head.

The surface tension of the ink should be selected so that a stable meniscus exists across the nozzle openings when droplets are not being ejected. A typical range of surface tension to meet this objective is 29-38 dyn/cm (mN/m). Examples of classes of solvents used in inkjet printer inks include acetates, alcohols, aldehydes, esters, ethers, glycols, glycol ethers, hydrocarbons, ketones and lactates. Commonly used solvents include acetone, diacetone alcohol, ethanol, ethyl acetate, ethyl lactate, ethylene glycol, diethylene glycol, butyl acetate, butyl lactate, benzyl alcohol, methyl ethyl ketone, propylene, propylene glycol, methoxy propanol, methoxy propyl acetate and water.

In one convenient embodiment of the invention two ink reservoirs are employed, one to serve as the main ink container and the other to serve as an ink-feed supply adjacent to the print head. Preferably the supply reservoir is located at a higher level than the main reservoir so that ink can flow from the upper to the lower, for example by gravity, without the use of pumps. The main pumping requirement is thus merely for recycling ink from the lower reservoir to the upper reservoir. This can be achieved by a single pump, the type and size of which can be selected according to the specific flow requirements so as to give optimum operating conditions.

Each reservoir preferably includes a vent to the atmosphere so as to facilitate smooth flow of ink to and from the reservoir. The vent preferably includes an on-off valve which can be closed when the system is not in use. Such a valve helps to prevent loss of solvent and facilitates movement of the printer between uses, for example if it needs to be returned to a supplier for maintenance or repair.

The volume of the supply reservoir is preferably small relative to the total ink volume in the system. This ensures that the ink in the supply reservoir is regularly changed and thus avoids the need for stirring or agitation of its contents. In a particularly convenient version of the two reservoir embodiment of the invention the upper and lower reservoirs are both located at a lower level than the print head such that the height difference between their ink levels provides a "siphon" to draw ink continuously to and through the print head. This configuration offers several advantages. Ink circulation through the print head is achieved by the siphon effect alone.

The print head receives a constant flow of ink, ensuring that ink is always present to be drawn into the capillaries.

In this version of the invention the rate at which ink passes through the print head is determined by the difference in ink levels between the two reservoirs, i.e. the vertical height between the upper surfaces of ink in the respective reservoirs. The ink levels in the respective reservoirs can thus be chosen and controlled to provide the optimum flow rate for a given printing duty. A further important factor is the vertical height between the capillary channels in the print head and the upper level of ink in the upper reservoir. In order to ensure the reliability of the siphon effect this further height difference should be relatively small, typically in the range 0.5 to 4 inches (12 to 100 mm).

The absence of pumping action in the siphon section of the system provides a uniform and suitably low ink feed pressure in the print head, avoiding unwanted seeping of ink from the nozzles and avoiding the pulsing action associated with many pumps. The absence of moving parts in this siphon section also offers highly reliable operations. Preferably the level of the upper surface of ink in the upper reservoir is maintained at a steady level. This helps to ensure uniformity of ink flow around the system and thus uniformity of print quality. It can be achieved by the presence of level sensors in the reservoir and by controlled inlet and outlet flow rates.

It is desirable for the ink to be fed into each reservoir, especially the feed into the upper reservoir, at a low level and positioned such that incoming ink imparts a swirling action in the reservoir, thereby further assisting in keeping the solids content in suspension.

For purposes of control or monitoring of printing operations the printers according to the invention can be employed in association with one or more microprocessors, for example a programmable logic controller. Each microprocessor can be located alongside the printer or remote from it. h one convenient arrangement one microprocessor forms part of the apparatus as such and another microprocessor is located remotely. A remote location is beneficial in permitting a reduction in on-site inspection time by local personnel and in permitting several application stations to be monitored and controlled from a single point. The microprocessors can be configured to receive data electronically by such transmission routes as a direct wiring connection, dedicated telephone line, radio link or internet link.

The present invention is further described with reference to Fig. 1, which is a schematic view of one version of printer with a continuous flow system according to the invention. It is emphasised that the invention is not limited to this specific version and that not all of the components illustrated in this version represent essential features of the invention.

The illustrated system includes a print head 10 of a type marketed as a Trident Ultrajet". The print head 10 has multiple capillary channels each having an outlet nozzle 11. A primary reservoir 14 which serves as the main container for ink 15 has an ink inlet tube 16 and ink outlet tube 18, a magnetically-driven impeller 20, an ink level sensor 22, an exit mesh filter 23 and a valved vent tube 24 with a glass fibre filter 25. The ink inlet tube 16 includes a flow meter to monitor the rate of ink transfer and an on/off valve 13. The impeller 20 is designed and operated both to ensure efficient dispersion of any settled solids and to reduce any solid settlement out of the ink. The ink level sensor 22 comprises a reed switch and an associated magnet. The filter 25 has a small pore size

(0.2 μm) to avoid ingress of dust or other foreign particles from the atmosphere.

An ink feed pump 27 in the ink outlet tube 18 conveys ink containing suspended solids through an in-line static mixer 29 with fixed vanes 30. The purpose of the mixer 29 is to ensure complete dispersion of the ink solids. From the mixer 29 the line 18 continues to a secondary reservoir 32 located at a higher level than the primary reservoir 14 and having a significantly smaller volume. The height difference between the upper level of the ink in the two reservoirs 14 and 32 is indicated in the figure by AH.

The secondary reservoir 32 has an ink level sensor 33 (of the same type as sensor 22 in reservoir 14), an exit mesh filter 34 and an outlet line 35 leading to the print head 10. The volume of ink in the secondary reservoir 32 is preferably maintained at a constant level by the sensor 33 and ink feed pump 27. The line 35 includes a priming pump 40 and also includes a flow meter. The reservoir 32 has a valved vent tube 37, similar to the vent tube 24 of reservoir 14, with a glass fibre filter 38, similar to the filter 25 of reservoir 14 and again to prevent ingress of dust or other foreign particles from the atmosphere. When commissioning a new system or restarting after close-down, the valve 13 is opened and the pump 40 is used to prime the print head 10 with ink and to establish circulation of ink through the entire system, thereby to bring the contained solids into suspension and to keep them in suspension. Maintaining the suspension is assisted by the action of the impeller 20 and the static mixer 29. The ink circulation is continued at all times, for example between print runs or overnight, until the system is closed down for maintenance or decommissioning. For a prolonged break between print runs, for example overnight, the valved vent tubes 24 and 37 are closed to prevent ingress of contaminants.

Except during print runs the nozzles 11 are protected by a removable cap (not illustrated) to avoid ingress of dirt or evaporation of solvent. Before the start of a print run the priming pump 40 is operated to ensure the presence of ink in the nozzle capillaries and to eject any residues or air bubbles. The cap is then removed and the print head wiped to clean away any residual matter. After the print run the nozzles are again covered by the cap.

The relative disposition of the reservoirs 14 and 32 is such that because of the height difference AH, ink is siphoned through the print head 10. The siphon effect from the constant level source ensures that the print head 10 receives a constant flow of ink at a uniform and slightly negative ink feed pressure of about 2 in. water gauge (0.5 kPa).

After the initial priming, the pump 40 can be switched off, ink circulation being then achieved by the siphon effect and the action of the ink feed pump 27 alone, which makes for highly reliable operations.

The continuous and reliable ink flow is maintained throughout the printing operations, and is only terminated at the end of a printing campaign, or when the printer is closed down or decommissioned. Termination is effected by simply closing the valve 13, which also has the advantage of preventing ink from siphoning out of the secondary reservoir 32 when the system is not in operation.

Claims

We Claim:

1. A drop on demand inkjet printer comprising a secondary reservoir for ink containing suspended solids, a print head having an array of capillary channels each with an outlet nozzle, and controlled pulsing means associated with each of the capillary channels, in which the printer further comprises a continuous ink-flow subsystem for maintaining a continuous flow of ink from the secondary reservoir through the print head to inhibit settling of solids out of suspension.

2. A printer as claimed in claim 1, which includes a static mixer to assist in maintaining the solids in suspension or in re-dispersing any settled solids.

3. A printer as claimed in claim 2, in which the static mixer is selected from one or more of static mixers, direct drive impellers, indirect-drive impellers, and vibration inducers.

4. A printer as claimed in any preceding claim, which includes one or more filters to prevent oversize or foreign particles from getting into the ink from the atmosphere.

5. A printer as claimed in any preceding claim, in which a filter is located at the base of the reservoir, in a line leading to the print head or to the reservoir, in direct contact with the ink so as to extract oversize or foreign particles therefrom.

6. A printer as claimed in any preceding claim, which includes two ink reservoirs, one to serve as the main ink container and the other to serve as an ink-feed supply adjacent to the print head.

7. A printer as claimed in claim 6, in which the secondary supply reservoir is located at a higher level than the primary reservoir so that ink can flow from the secondary reservoir to the primary.

8. A printer as claimed in claim 7, in which the secondary and primary reservoirs are both located at a lower level than the print head such that the height difference (AH) between their ink levels provides a siphon to draw ink continuously to and through the print head.

9. A printer as claimed in any of claims 6 to 8, which includes an ink feed pump for recycling ink from the primary reservoir to the secondary reservoir.

10. A printer as claimed in any preceding claim, in which each reservoir includes a valved vent tube to the atmosphere so as to facilitate smooth flow of ink to and from the reservoir.

11. A printer as claimed in claim 10, in which each valved vent tube includes an on-off valve which can be closed when the system is not in use.

12. A printer as claimed in any preceding claim, in combination with one or more microprocessors for control or monitoring purposes.

13. A method of drop on demand inkjet printing employing an ink containing suspended solids, in which the ink is fed from a reservoir to a print head having an array of capillary channels each with an ejection nozzle and associated controlled pulsing means, wherein a continuous flow of ink from the reservoir is maintained through the print head to inhibit settling of solids out of suspension.

14. A method as claimed in claim 13, in which the ink is agitated to assist in maintaining the solids in suspension or in re-dispersing any settled solids.

15. A method as claimed in claim 13 or claim 14, in which the ink is filtered to prevent oversize or foreign particles from getting into the ink or to extract such particles therefrom.

16. A method as claimed in any of claims 13 to 15, in which ink is circulated between two ink reservoirs, one to serve as the main ink container and the other to serve as an ink-feed supply adjacent to the print head.

17. A method as claimed in any of claims 13 to 16, in which the supply reservoir is located at a higher level than the main reservoir so that ink can flow from the upper reservoir to the lower.

18. A method as claimed in any of claims 13 to 17, in which the supply and main reservoirs are both located at a lower level than the print head such that ink siphons continuously to and through the print head.

19. A method as claimed in any of claims 13 to 18, in which the respective ink levels in the two reservoirs are chosen and controlled to determine the rate at which ink passes through the print head.

20. A method as claimed in any of claims 13 to 19, in which the ink volume in the or each reservoir is maintained at a steady level.

21. A method as claimed in any of claims 13 to 20, in which the ink level in each reservoir is achieved by a combination of level sensors and controlled ink flow rate

22. A method as claimed in any of claims 13 to 21, in which ink is pumped to recycle it from the lower reservoir to the upper reservoir.